Currently, with the development of wireless communications, there is a high market demand for receiver-transmitter devices suitable for transmitting and receiving a large quantity of data and equipped with a wide frequency band. Such transceivers must be integrated in a chip of semi-conductor material. For this reason various types of receivers like those with direct conversion have been produced that enable the problems in the traditional super heterodyne receivers to be overcome. The latter preferably comprise a band-pass filter, a low noise amplifier, a notch filter or notch filter and a mixer.
The sensitivity of the receivers can be degraded by spurious out-of-band signals due to various mechanisms. In superheterodyne receivers, the image signal has the same frequency deviation as the desired signal in regard to the clock of the local oscillator and has to be rejected. This operation can be facilitated through the use of a notch filter with programmable band, as the frequency of the image signal is known.
In some transceivers the signal transmission and reception circuits work simultaneously; the power of the transmitted signal is much higher than the power of the received signal and the insulation of the transmitted signal from the received signal is limited at the radio frequencies. In particular, if the receiver circuit is the type with direct conversion, its sensitivity can be greatly reduced by the lost fraction or leakage of the signal transmitted that overlaps the signal received. Considering the frequency of the transmitted signal, which can vary in accordance to the selected channel, is known, the rejection of the transmitted signal can be improved using a notch filter with programmable band.
Currently there are various transceiver devices that comprise a notch filter. However, such circuitry presents several inconveniences linked to the presence of noise during signal reception and to a limited accuracy of the frequency calibration.
The most common solution adopted to lessen the losses of the transmitted signal is to attenuate them at the input of the receiver or after the first amplification stage by using radio-frequency filters. Seeing the offset between the reception and transmission frequencies is rather low, the filters to use must be highly selective. These filters cannot be integrated into the same chip of the transceiver device, and the presence of the filters outside the chip of the transceiver device makes the solution very expensive, with a high consumption of energy.
In US Patent Application Publication No. 2004/0219900, a notch filter with high Q factor is described which is integrated with a first and a second low-noise amplifier and is totally contained in an integrated chip; said filter is suitable for the image rejection in radio-frequency applications. The notch filter comprises two Q factor enrichment circuits that are combined together to generate a negative impedance such that it compensates the losses in the low Q factor inductors located on the chip. To improve the image rejection the notch filter uses a circuit of automatic calibration of the current that consists of an analog multiplier.